1. Field of the Invention
The present invention relates to adjustable dosage syringes and more specifically to self-zeroing syringes that can be used to administer multiple doses by single loading of the syringe.
2. Brief Description of the Prior Art
Adjustable dosage syringes have been used in the art for either adjusting the dosage for a particular administration of the contents of the syringes and/or for delivering multiple dosages of the contents of the syringe in multiple separate injections. For example, in veterinary use, the dosage of a particular drug administered to an animal may vary considerably with the weight of an animal. Thus, when a herd of livestock is injected with a drug, the careful loading of a syringe in the field is not always practical. It is, therefore, conventional to provide the veterinarian with a syringe capable of adjustment of the dosage delivered from that syringe. These syringes are normally disposable syringes, accordingly, the veterinarian can select the proper dose for an individual animal, rapidly set the syringe for delivering only that dose, inject the animal, and dispose of the syringe with the remaining drug content therein. Alternatively, when a drug is to be administered to a large number of animals, multiple doses for a number of animals may be contained in a single syringe. The veterinarian selects the proper dosage for each animal, rapidly sets that dosage on the syringe and serially injects the animals with the proper dosage.
Since a large number of animals are often treated in a short period of time, it is most important that the adjustable syringe be capable of rapid adjustment for dosage delivered can be accurate in the dosage. Further, since these syringes are normally disposable, it is imperative that they are of such construction that is inexpensive to manufacture. Further, it is conventional to supply the syringes and drugs as a total package from the manufacturer. Therefore, the adjustable syringes must be capable of delivering dosages of varying amounts, consistent with the weight of the animal, and the increment of dosage with which the syringe may be set is relatively small.
U.S. Pat. No. 4,275,729 describes an adjustable dosage syringe wherein the syringe plunger has sets of multiple closely spaced indentations along the length of the plunger, and a dosage selection ring disposed around the plunger. The inside circumference of the dosage selection ring has protuberances, which are registrable with and receivable in one of the indentations of the set of indentations as follows. When one protuberance is received in an indentation of the first set of indentations, the other protuberance is also received in the second set of indentations. According to this invention, the desired amount of the syringe content is discharged when the plunger is engaged by the plunger-receiving end of the barrel, which is at the opposite end of the barrel.
The above patent also describes a number of art-related patented syringes. U.S. Pat. No. 3,563,240 provides a syringe with a plunger having peripheral thread thereon, and a cooperating threaded nut. By threading the nut up and down of the syringe plunger, adjustable dosage of the drug contained in the syringe may be delivered by way of the nut limiting the depression of the plunger into the syringe barrel. However, manually threading the nut up and down the plunger is relatively time-consuming, particularly when the dosage requirement from animal to animal varies considerably and the threading of the nut must be accurately performed in order to ensure that the correct dosage is given. Manufacture of such a syringe is also expensive, since it requires manual manipulation to thread the nut on the plunger during the assembling process.
Efforts have been made in the art to overcome such disadvantages, notable among which is the known xe2x80x9csplit-ringxe2x80x9d syringe wherein the xe2x80x9cnutxe2x80x9d is releaseably hinged about the circumference thereof. Whereby, the nut can be opened, manually slid to the portion of the plunger desired for the appropriate dose, closed on the threads, and the appropriate dose discharged from the syringe. However, here again, this requires manual opening and closing of the hinged xe2x80x9cnutxe2x80x9d and can be time-consuming for the reasons expressed above.
Older approaches in the art avoided some of the problems discussed above by providing a plunger with various types of replaceable stop means, e.g., pins, clips and the like, but these approaches were too cumbersome for field use. Another approach in the older art was that of providing indentations in a ring disposed about the plunger whereby the indentations would allow the ring to be lifted and, moved along the plunger to discrete recesses in the legs of the plunger. The ring could be dropped into and set the dosage discharged by the plunger. U.S. Pat. No. 2,856,925 is representative thereof. This older approach, while being relatively rapid to operate, suffered from the disadvantages that the dosage was fixed by the size of the recesses in the legs of the plunger. Consequently, the necessary clearance in the recesses for allowing the ring to drop therein could not accurately set the dosage discharged from the syringe.
Accordingly, while the advances in the art have improved the accuracy of the dosage which can be delivered, i.e., by virtue of the threaded nut arrangement and the like, these advances have been accompanied by less convenient and rapid use of the syringes. It would, therefore, be of considerable advantage in the art to provide an adjustable dose syringe which can not only be rapidly changed to set the dosage delivered but can very accurately set that dosage and provide for very small increments of dosage adjustment.